Electricity

Question 1

State what is meant by an alternating current

Answer 1

a.c. is a current which changes direction and instantaneous value with time.

Question 2

How do you determine the peak voltage from an oscilloscope?

Answer 2

Vpeak = number of divisions X y gain

Question 3

State what is meant by a direct current

Answer 3

d.c. is a current that does not change direction or instantaneous value with time.

Question 4

Sketch a graph of current vs time for a discharging capacitor

Answer 4

Discharging capacitor graph current vs time

Question 5

Describe the effect of changing resistance on the charging and discharging curves for a capacitor?

Answer 5

Increasing resistance increases discharge time

Question 6

State what is meant by the short circuit current

Answer 6

The current drawn from a supply when the terminals are connected together with no load resistance. This can produce a large current and can be dangerous.
I = E/r

Question 7

What is a p-n junction?

Answer 7

A p-n junction is an interface or a boundary between two semiconductor material types, namely the p-type and the n-type, inside a semiconductor. The p-side or the positive side of the semiconductor has an excess of holes and the n-side or the negative side has an excess of electrons.

Question 8

Describe experiments to measure the current and voltage for charging and discharging capacitors

Answer 8

A = Discharging B = charging
Measure the current and voltage at regular intervals using the voltmeter and ammeter

Question 9

What are the rules for a parallel circuit?

Answer 9

Is = I₁ + I₂… (splits at branches)
Vs=V₁ =V₂ =……
1/RT = 1/R1 + 1/R2 + …..

Question 10

State the difference between and intrinsic and extrinsic semiconductor?

Answer 10

Intrinsic = pure semiconductor
Extrinsic = impurities atoms have been added

Question 11

What is internal resistance?

Answer 11

Charges passing through a cell meet resistance. A cell therefore has internal resistance.

Question 12

How do you find the EMF and short circuit current from the graph?

Answer 12

EMF = y intercept
Short circuit current = x intercept

Question 13

State what is meant by lost volts

Answer 13

Vlost = Ir
The voltage across the internal resistance when a load resistance is connected.

Question 14

Describe an experiment to find the EMF and internal resistance of a supply.

Answer 14

Adjust variable resistor and take readings of V and I.
Plot a graph of V against I. Gradient of graph = -r.
OR
measure the voltage E when switch open. Close the switch and take a reading of V and I.
Calculate r using E = V + Ir

Question 15

State the two types of conduction in semiconductors?

Answer 15

Electrons moving in the conduction band Holes moving in the partially filled valence band

Question 15

State the two types of conduction in semiconductors?

Answer 15

Electrons moving in the conduction band Holes moving in the partially filled valence band

Question 16

Which equations are used for potential divider circuits?

Answer 16

V1 = (R1/ R1 + R2) x Vs
V1/V2 = R1/R2

Question 17

State what is meant by terminal potential difference (TPD)

Answer 17

Vtpd = E- Ir
The energy per coulomb
transferred by a supply when a load resistance is connected.

Question 18

How are peak and rms values related?

Answer 18

Vrms = V_peak/√2

Irms = I_peak/√2

Question 19

What is Ohm’s Law?

Answer 19

V=IR

Question 20

What is forward bias?

Answer 20

Forward biasing means putting a voltage across a diode that allows current to flow easily

• Current flows • Depletion
zone narrows

Question 21

Describe the band structure of insulators, semiconductors and conductors

Answer 21

The band gap in conductors overlaps. The band gap in semi conductors is small. The band gap in insulators is large.

Question 22

State what is meant by E - the electromotive force (EMF)

Answer 22

The total amount of energy given to each coulomb of charge by a power supply.

Question 23

What do the symbols mean? What are their units? Q=It

Answer 23

Q = Charge - measured in coulombs (C)

I = Current - measured in amperes (A)

t = time - measured in seconds (s)

Question 24

Describe how an n-type semiconductor is formed

Answer 24

A material such as silicon has impurities added with an extra outer electron e.g. Arsenic

Question 25

How do you find the internal resistance from the graph

Answer 25

The magnitude of the gradient

Question 26

What is reverse bias?

Answer 26

reverse biasing means putting a voltage across a diode in the opposite direction. The voltage with reverse biasing doesn't cause any appreciable current to flow. This is useful for changing AC current to DC current. No current • Depletion zone widens

Question 27

Silicon can be doped with arsenic to produce an n-type semiconductor. State the effect that doping has on the conductivity of silicon.

Answer 27

Increases conductivity.

Question 28

How do you calculate electrical power?

Answer 28

P = IV = I²R = V²/R USING RMS VALUES

Question 29

How do you determine the frequency from an oscilloscope?

Answer 29

T = divisions x timebase frequency = 1/𝑇 Remember: ms = miliseconds

Question 30

Describe how a p-type semiconductor is formed

Answer 30

A material such as silicon has impurities added with one fewer extra outer electron e.g. Indium

Question 31

State what is meant by a capacitance of 1F

Answer 31

1F means one coulomb of charge stored per volt

Question 32

What are the rules for a series circuit?

Answer 32

``` Is = I1 = I2...... Vs = V1 +V2 + ...... RT = R1 + R2 + ...... ```

Question 33

Explain what is meant by a photovoltaic effect

Answer 33

* Photons are absorbed when electrons move from the valence band to the conduction band. * The p-n junction causes the electrons in the conduction band to move towards the n-type semiconductor * A potential difference is produced across the junction

Question 34

C = ^Q/_V What is meant by the symbols What are their units

Answer 34

C= capacitance(F) Q= Charge(c) V=Voltage/Potential Difference(V)

Question 35

Explain why the reading on the voltmeter decreases when the switch is closed

Answer 35

There is a current flowing when the switch is closed and voltage is lost across the internal resistor.

Question 36

Which equations do you use to find the energy stored by a capacitor.

Answer 36

E=1/2 QV = 1/2 CV² = 1/2 Q²/C